Conventional working robots include a welding robot which performs arc welding to join a plurality of work pieces with each other. To control the welding robot, so called teaching-and-playback method is often employed. In this method, a path along a planned welding line on the work is preset by a human operator as instruction information for the welding torch to follow, and this instruction information is utilized for the welding operation performed by the welding torch (JP-A 7-104831 gazette). Another method is to use laser sensors to sense welding positions whereby a path along a planned welding line on the work is preset as instruction information for the welding torch to follow, and this instruction information is utilized for the welding operation to be performed by the welding torch.
However, in e.g. arc welding which is performed by using a welding torch, it is common that the welding line on the work becomes slightly different topographically or otherwise, before and after the welding. In fact, there are slight differences even between before and during, as well as during and after the welding operation. FIG. 12 shows how a welded region changed in the direction of its height before, during and after an arc welding operation. The measurements were made as the welding torch was moved. As shown in the figure, during the welding, the welding heat causes surface shape change in the work and therefore the surface of the work is formed slightly higher than before or after the welding. Thermal shape change in the work is known to be reproducible if the shape, thickness, and the method of fixation of the work are known.
The welding method disclosed in the above-mentioned gazette does not take into account the shape change in the work during the welding caused by heat. Therefore, even if a plurality of instruction points are set accurately based on pre-welding and post-welding surface conditions, and the welding is made along the path defined by the instruction points, there is still a problem that the instruction points do not follow the shape change on the work surface during the welding, and the level of welding is not satisfactory. For example, the welding torch will enter a bulged molten during the welding, which can result in defective welding. Especially when the work is a thin plate, the work surface shape change during the welding is relatively large, which means that probability for defective welding increases substantially.
In order to tackle with this problem, conventional methods use detection of welding results during the welding so that correction can be made to the instruction points in the manner of trial-and-error. As another attempt, fixing jigs are used to fix the work very tightly. However, the method of changing instruction points in trial-and-error is heavily dependent upon the skill, experience and so on of the human operator, which means quite a few failures must be expected before the operator understands how a particular work will change. As for the other attempt, fixing the work very tightly is often very difficult if the work has a thickness of e.g. 1 mm or less, because excessive localized shape change will occur in such an extremely thin piece.